1. Field of the Invention
This invention relates to a stretch blow molding machine for manufacturing plastic articles. More particularly, this invention relates to an apparatus which can continuously perform injection molding of a synthetic resin flat preform and stretch blow molding to create wide mouth cup-like containers.
2. Description of the Prior Art
An injection stretch blow molding machine is disclosed in U.S. Pat. No. 4,105,391. It comprises stages for injection molding of a preform, bringing the preform to a temperature suitable for stretch blow molding, stretching the preform into a blow mold and air-blowing the preform into a container such as a bottle, and removing the container. A neck mold or lip mold, which forms part of an injection mold, also serves as a holder. The mold moves together with a periodically rotating disk, transferring the preform and container from one stage to another.
In the aforementioned molding machine, the preform, which serves as a neck portion of a container, is transferred with an upper end thereof held on a neck mold and then subjected to stretching blow molding. However, where one is making a cup-like container which has no neck portion as in other containers such as bottles, it has been extremely difficult to perform injection stretch blow molding using conventional molding machinery, unless some circumferential portion of the preform is held by some means.
Generally, the means for holding a flat injection molded preform is designed so that an undercut groove is formed in an inner peripheral portion of a securing mold corresponding to a neck mold, and a preform peripheral edge serving as an open edge of a cup-like container is formed by this groove.
However, there is a disadvantage in forming such a groove in a securing mold. In addition, an undercut cannot be formed deeply to a sufficient extent even where the securing mold is split to remove an open edge from the groove, when the container is released, for example. Therefore, the peripheral edge of a preform occasionally slips partly out of the groove, as a result of the pressure produced when the preform is stretched by a stretching rod or when compressed air is introduced in the mold, causing a failure in the molding process.
Furthermore, where the preform is held by an undercut, protruding lines corresponding to the undercuts are formed annularly on the inside or outside or on both surfaces of the open edge. These lines create a difficulty in bonding a sealing material to the open edge.
Moreover, since the preform is held by the undercut, the material cannot be released unless the holder is divided. Because of the previously insurmountable difficulties in creating an alternative method of holding the preform, apparatus equivalent to container molds are presently used.